Game pieces known as “pull-tabs” generally contain two layers of paper. The game results are printed on a base layer and are temporarily obscured by a cover layer, which can be at least partially retracted (e.g., peeled away) to reveal the underlying game results.
U.S. Pat. No. 6,543,808 filed in the name of one of the present co-inventors discloses a direct thermal printable pull tab in which a thermally imageable layer is supported on a transparent substrate. An opaque coating or an opaque substrate covers the imageable layer, and an opaque cover substrate covers the transparent substrate. The imageable layer is thermally imageable through the opaque coating or the opaque substrate that covers the imageable layer. The cover substrate is at least partially removable or retractable to reveal the thermal imaging through the transparent substrate.
US Patent Application Publication No. 2003/0207066 to Killey discloses a thermally printed ticket structure in which a base substrate supports a thermal imaging material and a polymeric film containing an obscurant, such as vaporized aluminum, covers the thermal imaging material. An adhesive layer bonds the polymeric film to the base substrate, but a portion of the adhesive layer is covered by a release coating that allows the polymeric film to be retracted from the base substrate. Thermal images are formed in the thermal imaging material through the polymeric film. At least one area of the polymeric film overlying the release coating is retractable from the base substrate to reveal the thermal images.
We have found thermal printing through intervening layers to be problematic, particularly through layers of ink or adhesive layers in combination with opaque substrates. High concentrations of heat associated with thermal printing can leave trace images of the otherwise hidden printed matter on the surfaces of the obscuring layers, particularly where obscuring inks are used to hide the printed matter. Where opaque substrates are used, adhesive layers that bind the substrates together can interfere with the required transmissions of heat to the thermally printable medium or react adversely with the thermally printable medium, limiting the effectiveness or shelf life of the print medium. The high concentrations of heat required to effectively penetrate the opaque substrate and adhesive layers can also leave trace images of the printing on exposed portions of the constructs.